Crane safe load indicator

ABSTRACT

A crane safe load indicator for determining the safe load supported by a pivoted boom which can be luffed. The indicator functions by comparing a signal representative of the total turning moment of the boom about its pivot in terms of the angle included between the boom and a hydraulic ram which is supporting it, with a reference signal which is representative of the maximum safe turning moment of the boom about its pivot for the boom luff angle and load radius currently obtaining. A modification of the indicator provides a safe load indication when a fly jib is mounted at the end of the boom. In this modification account is taken of the change in centre of gravity of the boom fly jib and load due to change in boom extension.

United States Patent [191 Hutchings I CRANE SAFE LOAD INDICATOR [75] Inventor: Bernard David Francis Hutchings,

Cambridge, England [73] Assignee: Pye Limited, Cambridge, England 22 Filedz June 22, 1972 [21] Appl. No.: 265,329

[30] Foreign Application Priority Data June 25, 1971 Great Britain 30024/71 [52] U.S. Cl. 212/39 R, 235/1S1.33, 340/267 C [11] 3,870,160 [451 Mar. .11, 1975 1,000,613 8/1965 Great Britain 212/39 Primary Examiner-Evon C. Blunk Assistant Examiner-Jeffrey V. Nase Attorney, Agent, or Firm-Frank R. Trifari; Bernard Franzblau [57] ABSTRACT A crane safe load indicator for determining the safe load supported by a pivoted boom which can be luffed. The indicator functions by comparing a signal representative of the total turning moment of the boom about its pivot in terms of the angle included between the boom and a hydraulic ram which is supporting it, with a reference signal which is representative of the maximum safe turning moment of the boom about its pivot for the boom luff angle and load radius currently obtaining. A modification of the indicator provides a safe load indication when a fly jib is mounted at, the end of the boom. In this modification account is taken of the change in centre of gravity of the boom fly jib and load due to change in boom extension.

10 Claims, 7 Drawing Figures PATENIEUHARI 1 1975 SHEET 1 [IF 5 PATENTED 1 5 SHEET 2 [1F 5 PATENIED MARI 1 I975 sum 3 UP 5 Fig.5

PATENIEDHARI 1 I975 saw u or 5 Fig.6a

SHEET 5 9 5 CRANE SAFE LGAD ENDTCATOR This invention relates to a load indicator arrangement for use with cranes, derricks and other lifting apparatus of the type having a pivoted boom which can be luffed by an hydraulic ram or other boom supporting means.

A typical crane of the above type has a boom comprising a plurality of telescoping sections, of which the lowermost is pivoted to a base unit for luffing movement by means of a hydraulic ram which is also pivoted at one end to the base unit and has its other end pivoted to a point on the lowermost section so as to support the boom at an angle (the luff angle) to the horizontal which is determined by the extension of the ram. The base unit is normally arranged to slew through the whole or part of a circle about a vertical axis. As an alternative to the hydraulic ram, the boom can be supported by a winch cable which is secured to its outer end and which can be wound in and out to luff the boom. For this alternative, the boom is not usually telescopic.

In a basic mode of operation of the crane, a load is supported by a hoist rope passing over a sheave at the outer end of the boom. The crane can lift loads located within a range of radii measured from its slewing centre.

In general, where the boom is telescopic, any one of a plurality of combinations of boom extension and luff angle may be employed to bring the sheave vertically above a load located at a given radius from the slewing centre. In the case of a non-telescopic boom, different radii can be achieved only by varying the luff angle.

The maximum permitted (safe) load at a given radius is a function of the crane design. The crane manufacturer provides a rating curve or table which relates the maximum safe load for each value of radius. In the case of a telescopic boom, the rating curve or table also relates maximum safe load to boom extension for each value of radius because the maximum safe load will vary with boom extension.

For the handling of relatively light loads it is the practice to secure an extension or fly jib to the head of the outer section of the boom and to pass the hoist rope over a sheave at the outer end of the fly jib. This permits the crane to reach loads at a greater radius than when operated in the basic mode. Generally, the fly jib is of relatively light construction compared with the boom so that the latter can support any load which may be imposed on it by the fly jib irrespective of its extension (in the case of a telescopic boom) and luff angle. The maximum safe load for fly jib operation is thus a function of the design of the fly jib and varies with the angle which the fiyjib makes with the horizontal. Since the fly jib is secured at a fixed angle with respect to the axis ofthe boom, the maximum safe load for fly jib operation is related to the boom luff angle. The crane manufacturer also provides a rating curve or table relating maximum permitted (safe) load for fly jib operation to luff angle.

It is an object of the present invention to provide a crane or other lifting apparatus of the above type with a safe load indicator for indicating the actual load supported by the crane relative to the maximum safe load and for providing a warning signal if the actual load equals or exceeds the maximum safe load both when the crane is operated in the basic mode and when it is operated using an extension or fly jib.

According to a'first aspect of the present invention a safe load indicator arrangement for use with a crane or other lifting apparatus of the type specified comprises means for producing a signal representative of the actual total turning moment of the boom about its luffing pivot in terms of the angle included between the boom supporting means and the boom and of the reaction sustained by the boom supporting means in supporting the boom and any load suspended from it, means for producing a reference signal representative of the maximum safe turning moment of the boom about its pivot for the boom luff angle and load radius currently obtaining, and means responsive to these signals to produce a resultant output signal which can be utilised to provide an indication of the available lifting capacity.

The word reaction"is used herein to signify the force to which the boom supporting means is subjected in supporting the boom (and load). If the boom supporting means is an hydraulic ram then the force would be a function of the fluid pressure in the ram, whereas if the boom supporting means is a winch cable then the force would be a function of the strain to which the winch cable is subjected. The reaction sustained by the boom supporting means can thus readily be determined as an electrical signal by a pressure transducer or a resistance strain gauge transducer which is appropriately mounted to suit the crane design.

In carrying out the invention according to this first aspect thereof the load indicator arrangement can comprise transducer means for producing a first signal which is a function of the reaction sustained by the boom supporting means in supporting the boom and any load suspended from it, angle sensing means for modifying said first signal in accordance with the sine of the angle included between the boom supporting means and the boom to produce a first resultant signal which is representative of the actual total turning moment of the boom about its pivot, boom angle sensing means for producing a second signal which is proportional to the cosine of the boom luff angle, boom length sensing means for modifying said second signal in accordance with the boom length to produce a second resultant signal which is representative of load radius, load/radius law generator means adapted for operation in accordance with the load/radius rating of the crane and responsive to said second resultant signal to produce a reference signal which is representative of the computed total turning moment at the boom pivot for maximum safe load at the radius represented by said second resultant signal, and means responsive to said first resultant signal and said reference signal to produce an output in accordance with any difference therebetween and thus between actual total turning moment and maximum safe computed turning moment.

The invention according to this first aspect is applicable to to aforesaid basic mode of crane operation.

According to a second aspect of the present invention a safe load indicator arrangement for use with a crane or other lifting apparatus of the type specified and adapted for operation using a fly jib comprises means for producing a signal representative of the actual reaction sustained by the boom supporting means in supporting the boom and fly jib and any load suspended from the fly jib, means for producing a reference signal representative of the computed boom supporting means which would be required to sustain the boom supporting means at the boom luff angle currently obtaining with the maximum safe load for that luff angle suspended from the fly jib, and means responsive to these signals to produce a resultant output signal which can be utilised to provide an indication of the load.

In carrying out the invention according to this second aspect in the case where the boom is telescopic, means may be provided for modifying said referencesignal in accordance with boom extension because the signal indicative of boom supporting means reaction is also a function of boom extension in that it varies with a change in boom extension. Also, is is preferable in order to achieve greater accuracy of fly jib load indication to further modify said reference signal as a function of the shift of the combined centre of gravity of the boom fly jib and load due to a change in boom angle.

More specifically, the load indicator arrangement may comprise transducer means for producing a first signal which is a function of the reaction sustained by the boom supporting means in supporting the and fly jib and any load suspended from it, boom angle sensing means for producing a second signal which is proportional to the boom luff angle, first load/ angle law generator means adapted for operation in accordance with the fly load/boom angle rating of the crane and responsive to said second signal to produce a first reference -',signal which is representative of the boom supporting ,ineans reaction which would be required to sustain the "boom at maximum extension with the maximum safe fly jib load at the boom luff angle represented by said second signal, boom length sensing means for modifying said first reference signal in accordance with the boom length, second load/angle law generator means also adapted for operation in accordance with the fly load/boom angle rating of the crane and responsive to said second signal to produce a second reference signal representative of change in the centre of gravity of the boom, fly jib and load for change in boom angle, means for producing a resultant reference signal in response to the modified first reference signal and the second reference signal, and means responsive to said first signal and said resultant reference signal to produce an output in accordance with any difference therebetween and thus between the actual fly jib load and the maximum safe fly jib load.

In order that the invention may be more clearly understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG. I is a schematic drawing of a crane having a telescopic boom arranged for luffing;

FIG. 2 is a typical rating curve for the crane of FIG.

FIG. 3 is a schematic drawing of the crane of FIG. 1 with an extension or fly jib;

FIG. 4 is a typical rating curve for the crane of FIG. 3;

FIG. is a block schematic diagram of a safe load indicator arrangement according to the invention;

FIG. 6a is a block schematic diagram ofa law generator of the safe load indicator arrangement and FIG. 6b is a graph illustrating the operation of the law generator of FIG. 6a.

Referring first to FIG. 1, the crane there shown has a boom indicated generally by the reference numeral 1 and comprising a lower section 2, an intermediate section 3 slidable telescopically within the upper end of the section 2 and an upper section 4 slidable telescopically within the upper end of the section 3. An hydraulic extension ram, not shown in the drawing, is provided to vary the position of the section 3 with respect to the section 2 and a similar rampositions the section 4 with respect to the section 3, so that the overall length of the boom 1 may be adjusted to any desired value between a maximum and minimum limit.

The lower end of the boom section 2 is pivoted to a horizontal base unit 5 at a point 6 so as to permit luffing movement of the boom. An hydraulic luffing ram 7 (extensible boom support) has one end of its cylinder 8 pivoted to the base unit 5 at a point 9 and its piston rod 10, which extends through the other end of the cylinder 8, pivoted to the boom section 2 at a point 11. The Iongitudinal axis of the boom 1 makes an angle 6 (the luff angle) with the horizontal base unit 5, 0 being variable by varying the extension of the luffing ram 7. A load is suspended by a hoist rope 12 which passes over a sheave (not shown) provided at a point 13 at the outer end of the upper boom section 4 to a hoist mechanism (also not shown). The base unit 5 may be mounted by means not shown for slewing abou a vertical axis 14.

The crane is rated by themanufacturer in terms of the maximum safe loads which may be lifted at different radii 15 from the slewing centre 14. A typical rating curve 16 for a crane of the type described with reference to FIG. 1 is shown in FIG. 2. The load radius is variable between a minimum value 17 obtained with the luffing ram 7 fully extended and the boom sections 3 and 4 fully retracted and maximum value 18 obtained with the luffing ram 7 at a minimum extension and the boom sections 3 and 4 fully extended. Anintermediate radius such as 19 may be obtained by partially retracting the boom section(s) 3 and/or 4 while maintaining the luffing ram 7 fully extended, by partially retracting the luffing ram 7 while maintaining the boom sections 3 and 4 fully extended or by partially retracting both the luffing ram 7 and the boom sections3 and '4.

The crane as arranged for operation using an extension or fly jib will now be describedwith referenceto FIG. 3, in which the samereferences as in FIG. 1 are employed for integers common to the two FIGURES. A boom 1 comprising a lower section 2 and telescopically extensible sections 3 and 4 is pivotally mounted on a base unit 5 at a point 6 and is supported by an hydraulic luffing ram 7 having a cylinder 8 pivotally mounted on the base unit 5 at a point 9 and a piston rod 10 pivotally connectedto the lower-most boom section 2 at a point 11. As in FIG. 1, the base unit 5 is mounted for slewing about a vertical axis 14.

A fly jib 21 is secured to the outer end of the boom section 4 at pointl3. The fly jib 21 is shown at an angle [3 with respect to the longitudinal axis of the boom 1, such that the longitudinal axis of the fly jib 21 is inclined to the horizontal at an angle somewhat less than the luff angle 0, but the angle 3 may be zero, i.e. the fly jib 21 may be collinear with the boom 1. The hoist rope 12 now passes over the sheave provided at the outer end 22 of the fly jib 21.

As was stated previously, the strength of the boom and the stability of the crane as a whole are such that the maximum load which the fly jib 21 can impose at the point 13 can be sustained with any combination of boom extension and boom luff angle 0. The maximum safe load which may be supported by the hoist rope 12 is therefore determined by the strength of the fly jib 21 and the angle B) which it makes with the horizontal. Since B is constant the crane may therefore be rated in terms of boom luff angle 0. A typical rating curve 23 for a crane of the type described with reference to FIG. 3 is shown in FIG. 4.

A load indicator arrangement for use when the crane is adapted either for operation in the basic mode (FIG, 1) or for fly jib operation (FIG. 3) will now be described with reference to FIG. 5.

In this arrangement, a reference signal generator 24, for example, a 700 Hz square wave oscillator, provides a stable signal voltage V. The signal V is supplied to a transducer 25 which is connected to the luffing ram 7 and is adapted to produce an output signal P which is a function of the reaction sustained by the ram in supporting the boom (1) and any load suspended from it. When the ram 7 is of the single-acting type, the output of the transducer 25 is a function of (e.g., proportional to) hydraulic fluid pressure below the ram piston 10. For a double acting ram, the transducer output is a function of (e.g., proportional to) the difference between the pressures below and above the ram piston 10, modified by the ratio of the effective areas of the lower and upper sides of the piston.

The signal P is applied via an amplifier 26 to an input terminal of a ram angle sensor unit 27, comprising a potentiometer having a resistance track 28. The ends of the track 28 are connected to ground via respective resistances 29 and 30, and the signal P is connected to a tap point 31 intermediate the ends of the track 28. A slider 32 makes contact with the track 28. The potentiometer body is mounted in fixed relation to the boom land the slider 32 is mechanically coupled to the luffing ram 7 so that it moves over the track 28 when the angle (bincluded between the boom 1 and the ram 7 changes with changing extension of the ram. The track 28 is graded according to the sine law such that the signal M appearing at the slider 32 is proportional to the sin (1). The resistors 29 and 30 serve to limit the range of values of sin date that corresponding to the range of values of the angle permitted by the construction of the crane. Inspection of FIG. 1 will show that the actual total turning moment of the boom 1 about the pivot point 6 is balanced by the reaction of the ram 7 multiplied by sin (15. The signal M is therefore a function of this actual turning moment.

In an alternative arrangement of the ram angle sensor unit 27, the potentiometer is replaced by a plurality of fixed resistors connected in series, the junctions be tween successive pairs of resistors being connected to successive fixed contacts of a multi-way rotary switch. The relative values of the fixed resistors are chosen according to the sine law to produce a stepped potentiometer chain corresponding to the potentiometer track 28. The moving contact of the rotary switch is then equivalent to the potentiometer slider 32.

The output signal M produced by the ram angle sensor unit 27 is connected to a first fixed contact of a changover switch 33. The second fixed contact of the switch 33 is connected to the output of the amplifier 26 and the moving contact is connected to an input of a summing amplifier 34. The output of the amplifier 34 is connected to a meter 35 and to an input of an alarm unit 36.

The signal V produced by generator unit 24 is also fed to an input ofa boom angle sensor unit 37 comprising a potentiometer 38 having a cosine law and a potentiometer 39 having a linear law, connected in parallel between the signal line and ground. The potentiometers 38 and 39 are mounted for movement with the boom 1, and their respective sliding contacts 40 and 41 are gravity actuated so that there appears at the slider 40 a signal Vl proportional to the cosine of the angle 0 which the boom makes with the horizontal (the luff angle), and at the slider 41 a signal V2 proportional to the angle 0. Conveniently, the boom angle sensor unit 37 may comprise two inclinometers of the type described in our British Pat. No. 965,017.

The slider 40 is connected to a first fixed contact of a changeover switch 42, and the slider 41 is connected to an input ofa load/angle law generator unit 43, to an input of a load/angle law generator unit 44 and to a meter 45. The output of the unit 43 is connected to a second fixed contact of the changeover switch 42, and the moving contact of the switch 42 is connected to an input ofa boom extension sensor unit 46. Unit 46 comprises a potentiometer 47 and a fixed resistor 48. The slider 49 of the potentiometer 47 is coupled to the boom 1 so as to be driven from the end of potentiometer 47 adjacent resistor 48 to the opposite end as the boom extension is varied from minimum to maximum. Resistance values of the linear potentiometer 47 and the resistor 48 are chosen so that the total series resistance is proportional to the boom length when fully extended and the resistor 48 is proportional to the boom length when fully retracted. It will be appreciated that for applications in which the boom length is constant. the potentiometer 47 may be replaced by a fixed resistor.

The slider 49 is connected to the moving contact of a changeover switch 50. A first contact of the switch 50 is connected to a meter 51 and to an input of a load/- radius law generator unit 52. A second fixed contact of the switch 50 is connected to an input of a summing amplifier 53. The output of unit 44 is also connected to the input of the amplifier 53. g

Preferably, all of the changeover switches 33, 42, 50

and 54 are ganged together for operation so that the moving contacts of all the switches connect with their respective first fixed contacts or with their respective second fixed contacts.

The operation of the system is as follows:-

When thecrane is operated-in the basic mode (i.e., without the fiy jib 21), all of the switches 33, 42, 50 and 54 are set as shown in FIG. 5.

As described hereinbefore, the signal P which is a function of the reaction sustained by the luffing ram 7 in supporting the boom 1 and the load is fed to the ram angle sensor unit 27 where it is multiplied by the sine of the angle (1) included between the luffing ram 7 and the boom 1 to produce the signal M m P sin which is a function of the actual total turning moment of the boom 1 about the pivot 6. The signal M is fed to the input of the summing amplifier 34.

The boom angle sensor unit 37 produces an output V1 proportional to cos 0, where Bis the boom luff angle, which is fed to the extension sensor unit 46. The output unit 46 is a signal R proportional to the boom extension multiplied by cos 6, i.e. proportional to the load radius, i.e., the horizontal distance 55 (FIG. 1) between the load and the boom pivot point 6. The signal R is fed to the meter 51 which is suitably scaled to indicate the radius of the load from the slewing centre 14. The zero of the meter 51 is offset, either electrically or mechanically, by an amount corresponding to the distance 56 between the boom pivot point 6 and the slewing centre 14. The offset may be required to be of either sign, dependent on the design of the crane. In some designs the boom pivot is on the opposite side of the slewing centre of the-load as shown in FIG. l, in other designs it is on the same side.

- The signal R is also fed to the input of the load/radius law generator unit 52. This unit has a characteristic related to the crane manufacturers Load/Radius rating curve (16, FIG. 2) such that its output signal is proportional to the a computed turning moment at the boom pivot when lifting the maximum safe load at the radius corresponding to the input Signal R.

It will be appreciated that a different load/radius law generator unit is required for each different type and/or capacity of crane, to which differing manufacturesratings apply. For this reason, unit 52 is preferably arranged as a self-contained unit provided with means of connection to the rest of the equipment of a type, e.g., plug and socket connectors or terminal blocks, which permit of easy replacement. Similar considerations apply to the units 43 and 44. A more detailed description ofa typical law generator unit will follow hereinafter with reference to FIG. 6.

The output signal from the load/radius law generator 52 is fed via the changeover switch 54 to the input of the summing amplifier 34.

The signals reaching the input of the summing amplifier 34 are therefore a signal proportional to the actual total turning moment at the boom pivot and a reference signal proportional to the computed turning moment which would be produced by the crane lifting its maximum safe load at the same radius as the actual load. The gains of the various units of the equipment are selected so as to render the constants of proportionality the same for the two signals. These two signals also may be arranged to have opposite polarities, e.g., by arranging unit 52 to produce a complement signal.

When, therefore, the crane is lifting its maximum safe load, the net input to the amplifier 34 is zero, producing zero output, which is indicated at the calibration point of the scale of the load meter 35 (the meter zero being offset mechanically to this point during manufacture).

Increase of load above the rated maximum will produce a net input of one polarity and a corresponding output from the amplifier 34 which will drive the meter 35 to indicate in the overload region of its scale. Lesser loads will produce a net input'and corresponding output of opposite polarity from the amplifier 34, thereby driving the meter 35 into the safe region and thus indicating available lifting capacity.

The alarm unit 36 is arranged to give an audible and- /or visible alarm signal when the output of the amplifier 34 is zero or of the one polarity aforesaid, i.e., when the actual load equals or exceeds the'maximum safe load, and may also comprise a trip circuit to immobilise the hoist mechanism of the crane under these conditions.

When the crane is required to perform a fly jib operation, the changeover switches 33, 42, 50 and 54 are set to the position opposite to that shown in FIG. 5.

The signal P, which is a function of the reaction sustained by the luffing ram 7 in supporting the boom, fly

The output signal V2 of the boom angle sensor unit 37 is displayed by the angle meter and is also fed to the input of the load/angle law generator 43. This unit has a characteristic which is related to the manufacturers fly Load/Boom angle rating curve for fly jib operation (23, FIG. 4), such that the output of unit 43 is a signal which is of opposite polarity to the signal P and is proportional to a computed ram reaction which would be required to sustain the boom at maximum extension with the maximum safe load suspended from the fly jib at the boom luff angle represented by the output signal V2. v

The signal from unit 43 is fed via the switch 42 to the boom extension sensor unit 46, where it is modified to supply at the slider 49 a signal which is also a function of boom length, and which is fed via the switch 50 to the input of the summing amplifier 53.

A further signal is fed to the input of the amplifier 53 for the following reason. The reaction, sustained by the luffing ram when a load is supportedby the fly jib varies not onlywith boom angle and with boom extension, but also as a second-order function of load due to the position of the combined centre of gravity of the boom, fly jib and the load varying with variation of boom angle.

The output signal V2 of the boom angle sensor unit 37 is fed to the load/angle law generator unit 44 which has a characteristic related to the rated load/boom angle curve 23 and produces an output signal which is fed to the summing amplifier 53 and is proportional to the change in the ram reaction, for maximum rated load, due to the shift in the centre of gravity for change in the boom luff angle as represented by the output signal V2.

The two signals into the summing amplifier 53 are opposite in polarity to the signal P so that the output from the summing amplifier 53 is therefore exactly the complement of the. signal P when the maximum safe jib fly load is lifted at any boom angle and extension. This output signal is fed via the switch 54 to theinput of the summing amplifier 34.

The total signal input to the amplifier 34 when the system isadapted to a crane performing fly jib operation therefore varies in a manner similar to that described hereinbefore in relation to the system adapted to a crane performing the basic mode of operation.

Without the further signal from the unit 44, the maximum safe fly jib load would not be so accurately computed because, at'a given boom length, the ram reaction required to sustain the boom, fly jib and any load supported by it would be a miminum at maximum boom angle, but as the boom angle is decreased the reaction would not increase in direct proportion due to the change in the position of the combined centre of gravity. In fact, the reaction would be somewhat greater than a direct proportional increase. The actual reaction signal P includes this factor but the signal computed from the law generator unit 43 and modified by the extension sensor unit 46 does not and is some what less than it should be by a factor related to the change in the combined centre of gravity for a change in boom angle. This could give rise to a condition in which the output from the summing amplifier 53 is less than the signal P to produce a difference signal signifysimilar in that each is required to accept an input signal variable over a range of values and to provide an output signal related to the input signal according to a known law. In any given instance the law may not be a simple mathematical relationship, but will be expressed graphically in the form ofa curve relating values of input and output signals. The curve may be complex and include sudden changes or even reversals of slope.

The circuit arrangement shown in FIG. 6a is a typical example of a law generator and illustrates the generation of an output signal which is a complex function of an input variable.

An operational amplifier 55 has a feedback resistor 56 and input paths 5'76l inclusive, of which the paths 57-60 inclusive are connected to an input signal Vin, assumed in this instance to vary over the range V to +V. The input 61 is connected to a reference voltage source (-5V) via a present potentiometer.

Input path 57 comprises a resistor 62, so that the current l flowing through 62 to the input of the amplifier 55 (which input is a virtual ground) varies linearly with the input signal Vin as shown by the graph 63 in FIG. 66b.

Input path 68 comprises series resistors 64 and 65 with a switching circuit comprising the transistors 66 and 67 connected to their common point. The current i flowing through the path 58 to the input of amplifier 55 increases linearly with an increase ofthe input signal Vin until the potential at the emitter of transistor 66 causes the latter to conduct. Further increase of Vin thereafter increases the current through transistor 66. The point at which the transistor 66 first conducts is determined by the setting of the potentiometer 68 connected across a reference voltage supply.

The current l therefore increases linearly with an increase of the input signal Vin up to a point determined by the setting of the potentiometer 68 and is thereafter constant, as shown by the curve 69 in FIG. 6b.

The input path 59 comprises two series resistors and a transistor switching circuit similar to those of the path 58 and also an inverting amplifier 70. It therefore supplies a current I which increases linearly up to a value determined by the potentiometer 71 and is thereafter constant, but is of the opposite polarity to the currents I, and has shown by curve 72.

The path 60 comprises a series resistor and a series switching circuit. No current flows in path 60 until the input voltage reaches a value, determined by the setting of the potentiometer 73, at which the switching circuit commences to conduct. Thereafter the current Lincreases linearly with an increase of Vin, as shown by curve 74.

The path 61 comprises a series resistor connected to a source of constant voltage determined by the setting of the potentiometer 75.. The current in path 61 is therefore constant, as shown by curve 76.

The total input current I at the input of the ampliher 55 is the algebraic sun of the currents I, to I inclusive. varies with the input signal as shown by the curve 77.

The output voltage V of amplifier 55 is always such as to provide a current through the feedback resistor 56 equal and opposite to the input current I The current voltage V therefore changes with change of input voltage Vin according to the curve 77, but with opposite sign.

It is apparent that any required law can be approximated with a desired degree of accuracy by a suitable combination of input paths to an operational amplifier.

What we claim is:

l. A safe load indicator system for use with a crane or other lifting apparatus having a pivotally mounted boom and extensible boom supporting means comprising, means for producing a first signal representative of the acutal total turning moment of the boom about its pivot by determining the angle included between the extensible boom supporting means and the boom and the reaction sustained by the extensible boom supporting means in supporting the boom and a load suspended therefrom, means for deriving a second signal determined by the boom luff angle and the length of the boom and which is indicative of the current load radius, means responsive to said second signal for producing a reference signal representative of a maximum safe turning moment of the boom about its pivot and corresponding to the current load radius, and means responsive to the first and the reference signals for providing an indication of the available lifting capacity.

2. A safe load indicator system for use with a crane or lifting apparatus having a pivotally mounted extensible boom and extensible boom supporting means comprising, transducer means for producing a first signal which is a function of the reaction by the extensible boom supporting means in supporting the boom and any load suspended therefrom, angle sensing means for modifying said first signal in accordance with the sine of the angle included between the extensible boom supporting means and the boom to produce a first resultant signal which is representative of theactual total turning moment of the boom about its pivot boom angle sensing means for producing a second signal which is proportional to the cosine of the boom luff angle, boom length sensing means for modifying said second signal in accordance with the boom length to produce a second resultant signal which is representative of load radius, load/radius law generator means adapted for operation in accordance with the load/radius rating of the crane and responsive to said second resultant signal to produce a reference signal which is representative of the turning moment of the boom about its pivot for lifting a maximum safe load at the load radius represented by said second resultant signal, and means responsive to said first resultant signal and said reference signal to produce an output in accordance with any difference therebetween and thus between actual total turning moment and maximum safe turning moment.

3. A safe load indicator system for use with a crane or other lifting apparatus having a pivotally supported boom, a fly jib and extensible boom supporting means comprising, means for producing a signal representative of the actual reaction sustained by the boom extensible boom supporting means in supporting the boom, fly jib and any load suspended therefrom at the current boom luff angle, means for producing a reference signal representative of the computed boom supporting means reaction which would be required to sustain the extensible boom supporting means at the boom luff angle currently obtaining with the maximum safe load suspended from the fly jib for said luff angle and means i i responsive to said signals for providing an indication o the available lifting capacity.

4. A safe load indicator system as claimed in claim 3, wherein the boom is telescopic to vary its length, and further comprising means for modifying said reference signal in accordance with the boom extension.

5. A safe load indicator system as claimed in claim 4, further comprising means for further modifying said reference signal as a function of the shift of the combined centre of gravity of the boom fly jib and load due to a change in the boom angle.

6. A safe load indicator for use with a crane having a pivotally supported extensible boom, a fly jib and extensible boom supporting means comprising, transducer means for producing a first signal which is a function of the actual reaction force sustained by the extensible boom supporting means in supporting the boom, fly jib and any load suspended therefrom, boom angle sensing means for producing a second signal which is proportional to the boom luff angle, first load/angle law generator means adapted for operation in accordance with the fly load/boom angle rating of the crane and responsive to said second signal to produce a first reference signal-which is representative of the extensible boom supporting means reaction which would be required to sustain the boom at maximum extension with the maximum safe fly jib load at the boom luff angle represented by said second signal, boom length sensing dance with the fly load/boom angle rating of the crane and responsive to said second signal to rpoduce a second reference signal which is representative of a change in the centre of gravity of the boom, fly jib and load for a change in boom angle, means, for producing a resultant reference signal in response to the modified first reference signal and the second reference signal, and means responsive to said first signal and said resultant reference signal to produce an output in accordance with any difference therebetween and thus between the actual reaction of the extensible boom supporting means due to a fly jib load and the maximum safe computed reaction due to said fly jib load.

7. A safe load control system for a crane having an ture and extensible'boom supporting means for adjust- 12 ing the boom luff angle comprising, means for producing a first signal which is proportionalfto the reaction of the extensibleboom supporting means, means for modifying said first signal as a function of the sine of the angle formed between the boom and the extensible boom supporting means to produce a first resultant signal, means for producing a second signal which is a function of the cosine of the boom luff angle, means for mofifying said second signal as a function of the boom length to produce a second resultant signal indicative of the actual load radius, means responsive to said second resultant signal for computing a reference signal representative of the inaximum safe turning moment of the boom about its pivot and corresponding to the load radius indicated by said second resultant signal, and means responsive to said first resultant signal and said reference signal for indicating the available lifting capacity of the crane at the current load condition.

8. A control system as claimed in claim 7 wherein said first signal modifying means comprises a potenti-. ometer having a sine law resistance variation and a slider adapted to be coupled to the extensible boom supporting means to move proportional to the extension of said extensible boom-supporting means.

9. A control system as claimed in claim 8 wherein said second signal producing means comprises a second potentiometer having a cosine law resistance variation and a slider adapted to be coupled for movement with the boom and proportional to the boom luff angle and said second signal modifying means comprises a third potentiometer having a linear resistance variation and a slider adapted to be coupled to the boom for movement proportional to the boom extension.

10. A control system as claimed in claim 7 further comprising a first'switch for selectively connecting said first signal or the first resultant signal to an input of said indicating means, means for producing a-third signal which is a function of the boom luff angle, means responsive to said third signal for producing a second reference signal indicative ofthe computed maximum safe boom supporting means reaction at the current boom luff angle and which is in accord with the load/boom angle rating of the crane, and a second switch operated in synchronism with the first switch for selectively connecting said first and second reference signals to'an input of said indicating means. 

1. A safe load indicator system for use with a crane or other lifting apparatus having a pivotally mounted boom and extensible boom supporting means comprising, means for producing a first signal representative of the actual total turning moment of the boom about its pivot by determining the angle included between the extensible boom supporting means and the boom and the reaction sustained by the extensible boom supporting means in supporting the boom and a load suspended therefrom, means for deriving a second signal determined by the boom luff angle and the length of the boom and which is indicative of the current load radius, means responsive to said second signal for producing a reference signal representative of the maximum safe turning moment of the boom about its pivot and corresponding to the current load radius, and means responsive to the first and the reference signals for providing an indication of the available lifting capacity.
 2. A safe load indicator system for use with a crane or lifting apparatus having a pivotally mounted extensible boom and extensible boom supporting means comprising, transducer means for producing a first signal which is a function of the reaction by the extensible boom supporting means in supporting the boom and any load suspended therefrom, angle sensing means for modifying said first signal in accordance with the sine of the angle included between the extensible boom supporting means and the boom to produce a first resultant signal which is representative of the actual total turning moment of the boom about its pivot boom angle sensing means for producing a second signal which is proportional to the cosine of the boom luff angle, boom length sensing means for modifying said second signal in accordance with the boom length to produce a second resultant signal which is representative of load radius, load/radius law generator means adapted for operation in accordance with the load/radius rating of the crane and responsive to said second resultant signal to produce a reference signal which is representative of the turning moment of the boom about its pivot for lifting a maximum safe load at the load radius represented by said second resultant signal, and means responsive to said first resultant signal and said reference signal to produce an output in accordance with any difference therebetween and thus between actual total turning moment and maximum safe turning moment.
 3. A safe load indicator system for use with a crane or other lifting apparatus having a pivotally supported boom, a fly jib and extensible boom supporting means comprising, means for producing a signal representative of the actual reaction sustained by the extensible Boom supporting means in supporting the boom, fly jib and any load suspended therefrom at the current boom luff angle, means for producing a reference signal representative of the computed boom supporting means reaction which would be required to sustain the extensible boom supporting means at the boom luff angle currently obtaining with the maximum safe load suspended from the fly jib for said luff angle, and means responsive to said signals for providing an indication of the available lifting capacity.
 4. A safe load indicator system as claimed in claim 3, wherein the boom is telescopic to vary its length, and further comprising means for modifying said reference signal in accordance with the boom extension.
 5. A safe load indicator system as claimed in claim 4, further comprising means for further modifying said reference signal as a function of the shift of the combined centre of gravity of the boom fly jib and load due to a change in the boom angle.
 6. A safe load indicator for use with a crane having a pivotally supported extensible boom, a fly jib and extensible boom supporting means comprising, transducer means for producing a first signal which is a function of the actual reaction force sustained by the extensible boom supporting means in supporting the boom, fly jib and any load suspended therefrom, boom angle sensing means for producing a second signal which is proportional to the boom luff angle, first load/angle law generator means adapted for operation in accordance with the fly load/boom angle rating of the crane and responsive to said second signal to produce a first reference signal which is representative of the extensible boom supporting means reaction which would be required to sustain the boom at maximum extension with the maximum safe fly jib load at the boom luff angle represented by said second signal, boom length sensing means for modifying said first reference signal in accordance with the boom length, second load/angle law generator means also adapted for operation in accordance with the fly load/boom angle rating of the crane and responsive to said second signal to produce a second reference signal which is representative of a change in the centre of gravity of the boom, fly jib and load for a change in boom angle, means for producing a resultant reference signal in response to the modified first reference signal and the second reference signal, and means responsive to said first signal and said resultant reference signal to produce an output in accordance with any difference therebetween and thus between the actual reaction of the extensible boom supporting means due to a fly jib load and the maximum safe computed reaction due to said fly jib load.
 7. A safe load control system for a crane having an extensible boom pivotally mounted on a support structure and extensible boom supporting means for adjusting the boom luff angle comprising, means for producing a first signal which is proportional to the reaction of the extensible boom supporting means, means for modifying said first signal as a function of the sine of the angle formed between the boom and the extensible boom supporting means to produce a first resultant signal, means for producing a second signal which is a function of the cosine of the boom luff angle, means for modifying said second signal as a function of the boom length to produce a second resultant signal indicative of the actual load radius, means responsive to said second resultant signal for computing a reference signal representative of the maximum safe turning moment of the boom about its pivot and corresponding to the load radius indicated by said second resultant signal, and means responsive to said first resultant signal and said reference signal for indicating the available lifting capacity of the crane at the current load condition.
 7. A safe load control system for a crane having an extensible boom pivotally mounted on a support structure and extensible boom supporting means for adjusting the boom luff angle comprising, means for producing a first signal which is proportional to the reaction of the extensible boom supporting means, means for modifying said first signal as a function of the sine of the angle formed between the boom and the extensible boom supporting means to produce a first resultant signal, means for producing a second signal which is a function of the cosine of the boom luff angle, means for modifying said second signal as a function of the boom length to produce a second resultant signal indicative of the actual load radius, means responsive to said second resultant signal for computing a reference signal representative of the maximum safe turning moment of the boom about its pivot and corresponding to the load radius indicated by said second resultant signal, and means responsive to said first resultant signal and said reference signal for indicating the available lifting capacity of the crane at the current load condition.
 8. A control system as claimed in claim 7 wherein said first signal modifying means comprises a potentiometer having a sine law resistance variation and a slider adapted to bE coupled to the extensible boom supporting means to move proportional to the extension of said extensible boom supporting means.
 9. A control system as claimed in claim 8 wherein said second signal producing means comprises a second potentiometer having a cosine law resistance variation and a slider adapted to be coupled for movement with the boom and proportional to the boom luff angle and said second signal modifying means comprises a third potentiometer having a linear resistance variation and a slider adapted to be coupled to the boom for movement proportional to the boom extension. 